The present invention relates to a solar cell having a heterojunction formed from a crystalline silicon layer and an oxygen-added hydrogenated amorphous silicon thin film and a method for the manufacture thereof.
Solar light power generation technology is currently being developed with the aim of making solar energy one of the main energy sources of the 21st century. One example of this type of solar light power generation technology is the solar cell. Solar cells which use silicon, the most common semiconductor, as cell material are generally crystalline-based solar cells using single crystal silicon substrates or polycrystalline silicon substrates, as well as thin film-based solar cells such as amorphous silicon solar cells.
Amorphous silicon solar cells are usually low-cost solar cells because of the ease of increasing the cell area through the use of amorphous silicon thin films formed by applying a Chemical Vapor Deposition (CVD) process. However, with these type of cells, the present conversion efficiency is approximately a maximum of 12.5%, an efficiency lower than that of the crystal solar cell. As a matter of fact, the single crystal silicon solar cell has already achieved a conversion efficiency of at least 20%, and even the polycrystalline silicon solar cell has an efficiency of 16% while offering the advantage of using polycrystalline silicon which is cheaper than single crystal silicon.
Solar cells using a polycrystalline silicon substrate in which an n-layer is formed by diffusing phosphorus in to a p-type substrate are already known. However, such diffusion requires heating at elevated temperatures as high as 1000.degree. C. to 1100.degree. C., and the n-layer formed on the rear side must be removed by grinding, thereby causing disadvantages that require heat resistant manufacturing facilities and increased man-hours. The same holds true when single crystal silicon substrates are used.
As a result, a solar cell in which a p-n junction is formed by producing a thin film on a polycrystalline silicon substrate that has a conductivity type different from that of the substrate by means of a CVD process has been developed. Such a solar cell has been reported by H. Okamoto and Y. Hamanaka et al. in, for example, "Proc. 19th IEEE Photov. Spec. Conf., New Orleans (1987)", p. 689, and "Conf. Record 21st IEEE Photov. Spec. Conf., Florida (1990)", p. 1420. A cross-sectional construction of this solar cell is shown conceptually in FIG. 2 of the drawings. In reference to FIG. 2 regarding this type of solar cell, by applying an electron cyclotron resonance (hereinafter abbreviated as ECR) plasma CVD process, a p-n junction was formed by producing a p-type carbon-added hydrogenated micro-crystalline silicon (hereinafter known as .mu.C-SiC:H) thin film 22 with a thickness of 100 nm on an n-type polycrystalline silicon substrate 21 that has a resistivity of 356. cm, measures 5 cm.times.5 cm, and has a thickness of 0.3 mm. The material gas used in the CVD process was a mixture of SiH.sub.4, B.sub.2 H.sub.6, and H.sub.2. The substrate temperature was 300.degree. C., and the microwave power was 300 W. On the back of the polycrystalline silicon substrate, a rear electrode 3 was formed via the vacuum deposition of aluminum. On the surface of the p-layer 22 where the light shines, a transparent electrode 4 with an area of 0.08 cm.sup.2 is formed by mask-depositing zinc oxide (ZnO).
The compound, .mu.C-SiC:H, was used in the p-layer 22 to increase the cell output in two ways: 1) by increasing the light transparency resulting from the widened optical band gap, E.sub.g, in the p-layer on the side where light enters and, 2) by allowing the greatest possible amount of light to pass through the p-n junction by making the p-layer thinner than the diffusion layer. The output characteristics of a solar cell manufactured in this way was measured under a simulated solar light with an intensity of 100 mW/cm.sup.2. The measurement revealed that the open circuit voltage was 0.55 V, the short-circuit current density was 34 mA/cm.sup.2 the fill factor was 0 75, and the conversion efficiency was 14%.
It would be desirable to use the ECR plasma CVD process to form the p-type .mu.C-SiC:H thin film when manufacturing the above-described solar cell because the .mu.C-SiC:H thin film cannot be formed through a capacitive-coupling plasma CVD process, which utilizes a high-frequency voltage applied across the counter electrodes and which is suitable for the manufacture of large cells.
The generation of high-density plasma in ECR plasma CVD equipment is based on the following principle: electrons in a plasma, which is induced in a chamber in the magnetic field of a solenoid coil by a microwave guided to the chamber through a wave guide, are accelerated via resonance absorption of the microwave power in performing a cyclotron movement along the lines of the magnetic force, by which the density of the induced plasma becomes higher. Therefore, the available sizes for the solenoid coil and the microwave guide cause the film-formation area to be limited. This limitation makes the present process unsuitable for mass production and the enlargement in size of a heterojunction solar cell using crystalline-based silicon in the form of polycrystals or single crystals, although large substrates for them are available.
It is an object of the present invention to solve the aforementioned problems and provide a solar cell which can be produced in large quantities as well as in large sizes. It is a further object of the present invention to provide a method for manufacture of this type of cell wherein the cell is provided with a heterojunction formed from a crystalline-based silicon base layer and a silicon window layer with wide energy band gap formed thereon by means of a capacitive-coupling plasma CVD process.